In recent years, while various display members represented by liquid crystal displays (LCD) and plasma display panels (PDP) are being reduced in weight and cost, the market for optical films as important members of displays are being expanded. Optical films use various transparent films made of thermoplastic resins such as polyesters like polyethylene terephthalate (PET), acrylic polymers and polycarbonates (PC) as base materials, and in order to impart functionality to the base materials, for example, protective films (hard coating layers) for preventing defects, anti-reflection layers (AR layers), light-condensing/diffusing layers, polarizing sheets and the like are laminated on the base materials as various surface treatments, for obtaining the optical films. The base films are strongly demanded to be transparent. On the other hand, when the optical films are assembled into large displays and the like, thick films with thicknesses of 150 μm and larger are preferentially used since sufficient strength is requested.
A process for producing such a film includes the steps of extruding a molten thermoplastic resin from an extrusion die and cooling, and in order to produce an excellently transparent film, it is important to quickly cool the extruded thermoplastic resin to a desired temperature. A general method for quickly cooling a thermoplastic resin is a method of bringing the thermoplastic resin into contact with a cooling drum, and in the case where a thick film with a thickness of 150 λm or larger is produced, it is necessary to cool the film also on the film surface not kept in contact with the cooling drum (called the anti-cooling drum-side surface). The reason is that in the case where a thick film is cooled by a cooling drum only, the film temperature of the anti-cooling drum-side surface does not easily decline to a desired temperature. Especially in the case of a film made of a crystalline resin such as a polyester, unless the cooling rate near the glass transition temperature of the thermoplastic resin is high enough, the crystallization of the thermoplastic resin progresses to lower the transparency of the film as a result. As a method for quickly cooling the anti-cooling drum-side surface, known is a method of installing an auxiliary cooling device on the side of the counter-cooling drum-side surface, to promote the cooling of the aforementioned molten thermoplastic resin. The auxiliary cooling device is generally constituted by nozzles (hereinafter referred to as spray nozzles) for spraying cool air toward the thermoplastic resin on the anti-cooling drum-side surface (for example, patent document 1).
However, a thermoplastic resin, for example, a resin like a polyester has a nature that a low molecular weight substance such as an oligomer volatilizes from the surface of the molten resin film while the film is being cooled and solidified. For this reason, the oligomer volatilizing around the auxiliary cooling device is precipitated, for example, on the surfaces of the spray nozzles of the auxiliary cooling device, the surface of the exhaust mechanism, etc. The oligomer precipitated on the surface of the auxiliary cooling device may be guided by the air sprayed from the auxiliary cooling device, to fall and be deposited on the surface of the film, thereby making foreign matter defects. In the case where a distance is taken between the thermoplastic resin film and the spray nozzles in order to prevent that the oligomer precipitated on the surface of the auxiliary cooling device is deposited on the surface of the film, the cooling rate does not rise and the transparency of the film declines. On the other hand, if the tip faces of the spray nozzles are brought as close to the thermoplastic resin film as possible, the cooling rate rises, but the oligomer is precipitated on the auxiliary cooling device and sprayed to the thermoplastic resin film, to be deposited on the surface of the thermoplastic resin film, thereby causing foreign matter defects and other troubles in the subsequent steps.
To address such a problem, known is a method for producing a thermoplastic resin film, in which in the case where a molten thermoplastic resin is cast on a cooling drum, to be cooled and solidified on the cooling drum, for forming a thermoplastic resin film, an air supply means containing spray nozzles extending in the transverse direction of the aforementioned cooling drum is used to spray cooling air toward the film on the side of the anti-cooling drum-side surface of the film, while an inter-nozzle exhaust mechanism containing multiple shielding plates with exhaust holes formed therein is used for sucking and discharging the air near the film, wherein the air supply means and the exhaust means are alternately disposed in the film flow direction (for example, patent document 2).
Further, known is a cooling method, in which in the case where a molten thermoplastic resin is cast on a cooling drum, to be cooled and solidified on the cooling drum, for producing a thermoplastic resin film, spray nozzles of cooling air and the suction faces of an exhaust mechanism are alternately installed along the rotating direction of the cooling drum, wherein the ratio between the total amount of air sucked by all the suction faces and the total amount of air of all the spray nozzles is set at 3.4 to 4.5, in order to positively remove the air in the oligomer atmosphere. Furthermore, known is a technique in which a heater for preventing the precipitation of the oligomer is installed at least on the wall face inside the first suction face of the exhaust mechanism for the oligomer existing around the auxiliary cooling device (for example, patent document 3).
However, the method of positively discharging the air near the film by installing a suction/discharge mechanism in such a manner as to keep portions of the mechanism disposed between respective spray nozzles as described above not only has a problem of sucking the oligomer air near the film and volatilizing at the spray nozzle portions but also the following problem. If the temperature of the molten thermoplastic resin is higher, the oligomer is generated by a larger amount, to form an atmosphere having a high oligomer concentration in the proximity of the aforementioned extrusion die. Since the auxiliary cooling device exhausts a larger amount of air than a sprayed amount of air, highly concentrated oligomer air is also positively sucked. Therefore, as the cooling device is used for a longer period of time, the amount of the oligomer precipitated on the suction faces of the exhaust mechanism increases, and the precipitated oligomer falls on the surface of the thermoplastic resin film, to thereby cause foreign matter defects. As a result, the foreign matter defects increase with the lapse of time, to incur a loss due to the defects caused by use of the auxiliary cooling device, thereby lowering the productivity.
Further, in the case where heaters are installed inside the suction faces of the exhaust mechanism in order to prevent the precipitation of the oligomer on the suction faces, heating the suction faces to such a temperature as to prevent the precipitation of the oligomer is required to lower the cooling efficiency of the spray nozzles near the suction faces, thus causing cooling fluctuation and crystallization defects due to partial crystallization on the surface of the thermoplastic resin film. This, for example, raises the problem of degrading the transparency of the film required to be transparent. Further, even if the suction faces are heated to such an extent of not inhibiting the cooling function, the suction faces of the exhaust mechanism are very likely to have the oligomer precipitated and deposited thereon after use for a long period of time, and it is difficult to achieve both the requirements of sustaining the cooling efficiency and preventing the oligomer contamination.
As described above, the conventional methods did not allow an excellently transparent optical film to be produced while achieving both the requirements of sustaining the cooling rate and reducing the foreign matter defects caused by the dropping of the oligomer precipitated in the auxiliary cooling device.